Computers have traditionally interfaced with telephone company central offices over the plain old telephone service (POTS) network. A digital signal processor (DSP) within a computer sends information to the telephone company central office using a modem to transform the digital signals from the DSP into analog signals that can be transmitted over the POTS network. The modem also transforms analog signals received from the telephone company central office into digital signals for use by the DSP. The analog-to-digital and digital-to-analog conversions performed by the modems are accomplished with coder/decoders (CODEC). A CODEC is capable of interfacing with one DSP at a time. Therefore, interfacing multiple DSPs with a telephone company central office requires multiple modems or multiple CODECs within a modem.
FIG. 5 illustrates a typical interface between multiple data channels 2 and a TDM bus 8. This arrangement uses a separate modem 6 and control 4 for each data channel 2 which is to interface with the TDM bus 8.
Time division multiplexing (TDM) techniques are becoming increasingly common in telecommunication systems to increase the amount of information that can be carried on a transmission line. For example, TDM techniques are used in the internal architecture of private branch exchanges (PBXs) and in the transmission of digital signals over telecommunication lines to maximize the amount of data which can be handled by these systems.
The majority of contemporary telecommunication systems use a TDM arrangement in which each off-hook connection (i.e., when the telephone line is in use) is allocated a specific periodic time interval for information transfer. The periodic time interval is generally equal to eight times the data bit rate of the connected device, allowing a word (8 bits) of information to flow during each periodic time interval assigned to that device.
Conventional time division multiplexing (TDM) systems are designed to operate according to standard carrier TDM arrangements which have the capability of handling multiple channels on the same transmission line, such as T1 (24 channels), E1 (32 channels), 64-Slot (64 channels), and 128-Slot (128 channels). Each TDM arrangement consists of a fixed length frame used to transmit data. The frame is divided into a predetermined number of time slots, each representing a different channel. For example, a T1 line is designed to carry 24 voice-grade channels with data from each channel broken down into 8 bit words. Combining 24 voice channels (24 channels times 8 bits per channel equals 192 bits) into a serial bit stream and including a framing bit yields a frame size of 193 bits. E1, 64-Slot, and 128-Slot TDM arrangements operate according to similar principles, with the exception that a framing bit is not used.
As the requirements for bandwidth have increased, many companies now connect to the telephone company central offices digitally, rather than over the traditional POTS network. Companies are being connected to telephone company central offices with digital transmission lines such as T1 lines that utilize time division multiplexing (TDM) techniques which were previously used exclusively by telephone companies in the backbone between telephone company central offices.
Internet service providers (ISPs) offer Internet access to home users and small businesses. ISPs permit access to the Internet by allowing home users to call local telephone numbers and use modems connected to the user's computer to communicate with modems located at the ISP. The ISP then processes the information received at its modems to generate data streams which can be placed onto digital telecommunication lines, such as a T1 line, and transfers the data received by the modems located at the ISP to a telephone company central office for connection to the Internet. As the popularity of the Internet expands, ISPs will require telecommunication devices which allow data received from users through a large number of modem connections to be processed and placed on transmission lines that make the most efficient use of the transmission lines in a minimal amount of hardware space.